Phospholipid vesicles are being used as models to study the phenomenon of membrane fusion. An experimental project is proposed to examine the conditions and to understand the physical mechanism of fusion of acidic phospholipid vesicles induced by divalent cations. In the initial phase of this project, an anomalous temperature dependence in the extent of calcium-induced fusion of phosphatidylserine vesicles was discovered. Systematic measurements will be made on the extent and kinetics of fusion of different acidic phospholipids as a function of temperature and divalent cation concentration. The experimental approach is a novel combination of dynamic light scattering to measure vesicle size, stopped-flow rapid-mixing technique to study fusion kinetics, laser Raman spectroscopy to obtain molecular information, and diamagnetic susceptometry to study the bilayer phase transition. The results will be analyzed to determine whether the enhanced fusion is due to critical fluctuations near the membrane phase transition. The relation between calcium-induced fusion and molecular segregation in vesicles of mixed phospholipids will be studied. The long-term objective of the project is to increase our understanding on a molecular level of the role of divalent cations in regulating many membrane fusion processes in cellular and subcellular activities. The results will also be useful in the development of vesicle encapsulation systems in therapeutic medicine.